Download Examples Peer-to-Peer Applications

Examples Peer-to-Peer
Applications
(Gnutella, Kazaa, BitTorrent,
Skype)
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Second
generation
approach
Gnutella
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Fully distributed approach
Constructs Gnutella overlay network
What is an overlay network?
How do we join the network?
Query flooding
•  Performance limitations?
•  Easy to read specification
•  http://rfc-gnutella.sourceforge.net/src/rfc-0_6-draft.html
Gnutella: protocol
Query message
sent over existing TCP
connections
peers forward
Query message
QueryHit
sent over
reverse
Query
path
QueryHit
Scalability:
limited scope
flooding
File transfer:
HTTP
Query
QueryHit
Kazaa: Not all peers are equal
•  Basic idea
–  Peers and group leaders
•  TCP connection between peer and its group leader
•  TCP connections between some pairs of group
leaders
–  Group leader tracks the content in all its children
•  Speed ups
–  Limitations on simultaneous
uploads
–  Request queuing
–  Incentive priorities
–  Parallel downloading
•  Leads to next generation….
BitTorrent
Please read this short but in my estimation
brilliant paper:
http://www.bittorrent.org/bittorrentecon.pdf
The cool idea behind BitTorrent
•  Make downloading as fast as possible and
ignore search
•  Solve the freeloader problem and therefore
make downloading as fast as possible
•  Tit for tat protocol makes sure that
downloaders have to be uploaders too
•  Hot content gets quickly distributed in the
swarm
Piece Selection Strategies
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Strict priority
Rarest first piece
Random (really) first piece
Endgame
Choking Peers
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Choking algorithms
Opportunistic Unchoking
Anti snubbing
Upload only
Joining a torrent
Download Arctic_Monkeys-7cSm-2.torrent
Peer initiating a
new download
Web Server
Tracker
.torrent:
d8:announce30:http://tracker.prq.to/announce13:announce-listll48:http://open.tracker.thepiratebay.org:80/
:name48:Arctic_Monkeys-cSm12:piece lengthi65536e6:pieces13280
Existing Leach
Seed Peer
Fragment 5
Joining a torrent
Peer initiating a
new download
Web Server
Tracker
Connect to tracker. HTTP GET
Peer List.
Existing Leach
Seed Peer
Fragment 8
Data exchange
Peer initiating a
new download
Web Server
Tracker
Fragment 10
Fragment 1
Existing Leach
Seed Peer
Fragment 2
Data exchange
Peer initiating a
new download
Web Server
Tracker
Fragment 7
Fragment 10
Existing Leach
Seed Peer
Fragment 9
Algorithm: use hashes to verify pieces; download sub-pieces (fragments) in parallel;
look for rarest pieces first; advertise received pieces to the entire peer list
Peer-to-Peer Internet
Telephony Network
“Hello Mum!”
www.skype.com/
Client-side operations
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Login
User search
Start and end calls
Media transfer
Presence messages
• 
Skype client (SC)
Skype architecture
–  which is an application to start/receive calls, IM,
send/receive files
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Login server
PSTN
Super nodes (SN)
–  any node with sufficient CPU, memory, and network
bandwidth can become a SN.
–  A SC can become a SN - and can’t prevent this.
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SkypeOut
Login server (LS)
SkypeIn
–  stores user’s profile and performs users
authentication
–  Implements a public key infrastructure (PKI)
–  Infrastructure with clients
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Gateways
–  SkypeIn and SkypeOut servers provide PC-toPSTN and PSTN-to-PC bridging, respectively
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Tricks
–  Network Address Translation (NAT) and firewall
traversals are important Skype functions
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TCP for signaling, and both UDP and TCP for
transporting media - different ports for signaling
and data transfer
Wideband codec for 5-32 kbps
Unique skype user names across system
Ordinary Host (SC)
Super Node (SN)
neighbor relationship in the
Skype network
Observations
•  The problem is that there are a lot of different
configurations. Some routers allow outgoing connections
but not incoming. Some others allow UDP (User Datagram
Protocol) connections. Others allow TCP (Transmission
Control Protocol). Most existing Internet telephony
applications don't work that well in consumer
environments.
•  How does Skype get around that?
When both parties are behind NATs (Network Address
Translation), they can't actually set up a connection
between each other.
•  Use the overlay for connecting these peers
•  Setting up hot standby connections. Set up four, maybe
five standby paths.
•  There are a few hundred clients per supernode.
Bootstrapping into the system
PSTN
A SC builds and refreshes a
host cache (HC) which is a
list containing a set of SN’s
IP addresses-port number
tuples.
SN1
1
1
2
SC1 1
(1) 
(2) 
SC1 broadcasts a UDP lookup message to discover the closest
SNs
SN1 responds back to SC1 and SC1 puts SN1’s IP address and
TCP port number in the HC list (if SC1 does not get any response
back it tries to associate with one of the seven IP address port
pairs of SN provided at logon
Logging on to the system
Login
server
PSTN
2
3
SN1
1
SC1
(1) SC1 must establish a TCP connection and exchange information with SN1
in order to join the network. If the connection attempt fails the system returns
“Login failure”
(2) SN1 forwards the authentication request to the Login server
(3) The login server authenticates SC1 and responds to SC1
User search
•  Skype uses its proprietary Global Index
technology to performs users search
across the network in a scalable and
distributes fashion
Control messaging over the P2P
network
PSTN
SN1
TCP
SN2
SC2
SC1
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SCs sends control traffic including availability information, user search,
instant messages, and requests for VoIP and file transfer session over the
skype P2P network. The control messages are sent over TCP
Call setup with public IP addresses
PSTN
SN1
SC1
SN2
SC2
1 and 2 [TCP (signaling)]
3 [UDP (media stream)]
(1) Assume SC2 was in the buddy list of SC1 (if not run the
search over TCP). The caller (SC1) presses dial and attempts to
establish a TCP connection with the buddy SC2 to exchange
signaling information
(2) SC2 responds back and a TCP connection is established
(3) Media stream flows over UDP being encrypted
Another call setup scenario
The “caller” (who initiates the call) is behind NAT
(network address translator) and the “callee” (the
party called) has public IP addresses
Note, that all broadband users are
behind NATs and/or firewalls.
(1) Similar to the previous case but the
TCP signaling exchange does not flow
directly between the two SC 1 and SC 2
but rather through another intermediate
node (SN1, for example)
PSTN
SN1
1 [TCP]
SN2
NAT
(2) The media stream is transferred over
UDP being encrypted but again through
SN1
SC2
SC1
2 [UDP (media stream)]
through SN1, for example
Call setup when both caller and callee are
behind a NATs
PSTN
SN1
NAT
SC1
1 [TCP
(signaling)]
SN2
NAT
2 [TCP (media stream)]
SC2
(1) Similar to the previous case but the TCP signaling exchange does
not flow directly between the two SCs
(2) The media stream is transferred over TCP being encrypted
Call setup and teardown
•  For the call teardown the signaling flows over
TCP connections either directly (both caller
and callee having public IP addresses) or
indirectly if either of them is behind a NAT
(Network Address Translation - we will talk
about this later in the course).
Silence suppression
•  Skype uses efficient codec to encode voice  5
KBps total uplink and bandwidth usage
•  Skype doesn’t perform silence suppression
(interesting)
–  Cons: increase bandwidth usage
–  Pros: silence packets maintain UDP bindings at NAT and
when the media traffic is sent over TCP silence packets
avoid the drop in TCP congestion window size (which
otherwise would take several RTT to reach the maximum
level again)
Reverse engineering …
•  Skype is propriety system not fully
understood or documented, start here
–  http://en.wikipedia.org/wiki/Skype
–  there are a few papers that have reserved
engineered the signaling system; this is a good
one to read:
•  http://www1.cs.columbia.edu/~library/TR-repository/
reports/reports-2004/cucs-039-04.pdf
Be entrepreneurial – take risks
•  Niklas Zennstrom very cool entrepreneur and
co-inventor of kazaa, skype, joost
–  http://news.com.com/2008-7352-5112783.html
•  Like this Q&A snippet:
–  Are you afraid of intruders targeting your server that signs
user ID keys?
The signing server is like Fort Knox.
–  Where's it located?
I won't tell you. That's kind of a sensitive part of (the
company). It's very, very secure.
–  Message: while young be entrepreneurial, be a
risk taker!